German Edition: DOI: 10.1002/ange.201501880 Photoredox Catalysis International Edition: DOI: 10.1002/anie.201501880 Trifluoromethylchlorosulfonylation of Alkenes: Evidence for an Inner- Sphere Mechanism by a Copper Phenanthroline Photoredox Catalyst** Dattatraya B. Bagal, Georgiy Kachkovskyi, Matthias Knorn, Thomas Rawner, Bhalchandra M. Bhanage, and Oliver Reiser* Dedicated to Henri Brunner on the occasion of his 80th birthday Abstract: A visible-light-mediated procedure for the unprece- dented trifluoromethylchlorosulfonylation of unactivated alkenes is presented. It uses [Cu(dap) 2 ]Cl as catalyst, and contrasts with [Ru(bpy) 3 ]Cl 2 , [Ir(ppy) 2 (dtbbpy)]PF 6 , or eosin Y that exclusively give rise to trifluoromethylchlorination of the same alkenes. It is assumed that [Cu(dap) 2 ]Cl plays a dual role, that is, acting both as an electron transfer reagent as well as coordinating the reactants in the bond forming processes. The introduction of the trifluoromethyl group into organic molecules usually leads to a significant improvement of the chemical and metabolitic stability of drug candidates. [1–3] Numerous procedures, including nucleophilic, electrophilic, or radical approaches were developed for the installation of the CF 3 group, [4] including recent examples in which visible- light-mediated redox photocatalysis is used as key step. [5] Ruthenium- and iridium-based complexes can effectively catalyze the cleavage of CF 3 I, [6] Tognis reagent, [7] Umemotos reagent, [8] or triflyl chloride. [9] The role of these photoredox catalysts in such transformations is assumed in the transfer of an electron to the trifluoromethyl source by an outer-sphere mechanism, providing CF 3 radicals that undergo further transformations. Exploring the potential of [Cu(dap) 2 ]Cl [10] (À1.43 V vs. SCE; dap = 2,9-bis(para-anisyl)-1,10-phenanthroline) as a visible-light-driven photoredox catalyst, we report herein the unprecedented trifluoromethylchlorosulfonylation of unactivated alkenes, suggesting an inner-sphere mechanism that determines the outcome of the reaction beyond a photo- initiated electron transfer. In contrast, commonly used ruthenium or iridium complexes or eosin Y give rise to trifluoromethylchlorination with concurrent loss of sulfurdi- oxide, being in agreement with an outer-sphere electron transfer mechanism commonly assumed in photoredox cata- lyzed ATRA (atom-transfer radical addition) reactions. [11] Thermal cleavage of triflyl chloride catalyzed by [Ru- (Ph 3 P) 2 Cl 2 ] was demonstrated by Kamigata et al. [12] for the trifluoromethylchlorination of alkenes. Han et al. showed that the same process can be achieved by photoredoxcatalysis using [Ru(bpy) 3 ]Cl 2 (Scheme 1). [9b] In both cases the mech- anism is assumed to involve the oxidative Ru 2+ /Ru 3+ cycle, and electron transfer to triflyl chloride is accompanied with SO 2 extrusion. Following our development of ATRA reactions by visible light photocatalysis using [Cu(dap) 2 ]Cl, [13] we examined the feasibility of this catalyst towards trifluoromethylations of alkenes. Surprisingly, we observed that green light (LED 530 nm) irradiation of an acetonitrile solution of allylbenzene (1a) and triflyl chloride (2 equiv) in the presence of 1 mol % [Cu(dap) 2 ]Cl resulted in an ATRA process without SO 2 extrusion, giving rise to 2a in moderate yield (Table 1, entry 1). The expected product 3a was only observed in trace amounts. The presence of a base significantly improves the yield of 2a, which was isolated in 86 % (1 mol % [Cu(dap) 2 ]Cl) or 63 % (0.5 mol % [Cu(dap) 2 ]Cl) yield, respec- tively, upon addition of K 2 HPO 4 (entries 2,3). Variation of solvents (entries 4, 5) results in a slight decrease in yield; Scheme 1. Trifluoromethylations of alkenes by visible-light photocatal- ysis. [*] D. B. Bagal, Dr. G. Kachkovskyi, [+] M. Knorn, T. Rawner, Prof. Dr. O. Reiser Institut für Organische Chemie, Universität Regensburg Universitätsstrasse 31, 93053 Regensburg (Germany) E-mail: oliver.reiser@chemie.uni-regensburg.de D. B. Bagal, Prof. Dr. B. M. Bhanage Department of Chemistry, Institute of Chemical Technology Matunga, Mumbai-400 019 (India) [ + ] Current address: Institute of Bioorganic Chemistry and Petro- chemistry of National Academy of Science of Ukraine Murmanska str 1, 02660, Kyiv (Ukraine) [**] The authors thank the DFG (GRK 1626, Photocatalysis), the DAAD and the Humboldt foundation for financial support. We thank Sabine Stempfhuber, Universität Regensburg, for carrying out the X- ray crystal structure analysis of 5d, and Prof. R. D. Little, UC St. Barbara, for helpful discussions. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201501880. A ngewandte Chemi e 1 Angew. Chem. Int. Ed. 2015, 54,1–5  2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! Ü Ü